1. Field of the Invention
This invention relates to a liquid crystal display (hereinafter abbreviated to LCD) projection apparatus in which a video signal formed from the entering white light is separated into chrominance components of three primary colors, and then the components after having been modulated by using respective monochrome video image display devices such as LCD ones are synthesized again to permit projection of an image in full color.
2. Description of the Related Art
Recently, television sets of the projection type have rapidly come into such wide use that they can be seen not only in the public facilities but also in home. This kind of apparatus uses three high-luminance cathode ray tubes in which are made up color component images corresponding to the respective color component lights of red (R), green (G) and blue (B) and these images are projected by projection lenses onto a screen where they are synthesized to display a picture of the original color. FIG. 1 shows the outline of the video projection apparatus. 1, 2 and 3 are cathode ray tubes corresponding to the color components R, G and B respectively and are driven by drive circuits 4, 5 and 6 for R, G and B into which the video signals of the color components R, G, B enter respectively. 7, 8 and 9 are projection lenses, and are each arranged in focus on a screen 10 in front of the cathode ray tubes 7, 8 and 9. Note, in this figure, the projection lens is shown by a single lens, but in actual practice is usually constructed with a plurality of lenses for correction of various aberrations.
However, such an apparatus increases in size, and, when the distance to the screen is changed, re-adjustment is required so that the three monochrome projected images overlap one another on the screen.
Therefore, as an arrangement for the possibility of projecting by one projection lens P, what is shown in FIG. 2 is considered. In the figure, S is an illumination light source issuing white light, for example, for use in the Koehler illumination system. As means for separating parallel entering light A which is white light into three primary colors, use is made of dichroic mirrors 11 and 12 having two dichroic layers of different wavelength regions crossed to each other. For example, the dichroic mirror 11 reflects the blue color component B, and the dichroic mirror 12 reflects the red color component R. Of the three color components R, G and B separated by this crossed dichroic mirrors 11 and 12, the color component R is reflected by total reflection mirrors 13 and 14, the color component G goes straight as it is, and the color component B is reflected by total reflection mirror 15 and 16, entering LCDs 17, 18 and 19 corresponding to the respective ones. Because in the LCDs 17, 18 and 19, pictures of each color component of red, green and blue are imaged out, when light permeates here, the variation of transmittance due to the pictures of each of the LCDs 17, 18 and 19 is modulated to the variation of intensity of light.
That is, in FIG. 2, color components R, G and B of red, green and blue are modulated by the video signals of the LCDs 17, 18 and 19, respectively, becoming color components (color information light) R', G' and B'. These lights are synthesized again by a dichroic prism 20 to produce an exiting light A'. Note, the exiting light A' is, as known in the art, projected by the projection lens P onto the screen. On the screen, a picture of full color comes out. In such a manner, in the prior known apparatus, the two dichroic mirrors 11 and 12 are used in the crossed state. Therefore, a portion of the reflection surface of one dichroic mirror 11 is stripped off, giving a drawback that the picture is partly broken down. For example, in FIG. 3, on assumption that the dichroic mirrors 11 and 12 cross each other at right angles, the relationship between the thickness t of the dichroic mirror and the size t' of the broken portion of the reflection surface is expressed by the following equation: EQU t'=t/2.sup.0.5 . . . (1)
The value of t' of this equation (1) cannot be ignored, and a problem arises that by that portion alone, the center of, for example, the blue is broken down to a stripe shape. Also, the dichroic prism 20 used for synthesizing the modulated color components in FIG. 2 is very high in price, and its weight is also large, being unsuited to be used in home. Further, the optical path from the entering light A to the exiting light A' differs from color component to component. Therefore, the problem of aberration etc. also is large.